1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, an in-plane switching mode liquid crystal display device.
2. Discussion of the Related Art
Twisted nematic liquid crystal display devices (hereinafter TN LCDs) having high image quality and low power consumption are widely applied to flat panel display devices. TN LCDs, however, have a narrow viewing angle due to refractive anisotropy of liquid crystal molecules. This is because horizontally aligned liquid crystal molecules prior to applying a voltage become nearly vertically aligned with respect to the substrate when voltage is applied to the liquid crystal panel.
Recently, in-plane switching mode liquid crystal display devices (hereinafter IPS-LCDs) have been widely studied for improving the viewing angle characteristic. These liquid crystal molecules are nearly horizontally aligned.
FIG. 1a is a plan view of a unit pixel of a conventional active matrix LCD. As shown in FIG. 1a, unit pixel region is defined by a gate bus line 1 and a data bus line 2 in which the lines 1, 2 are arranged perpendicularly and/or horizontally in a matrix on a transparent substrate (first substrate) 10. A common line 3 is arranged parallel to the gate bus line 1 in the pixel region. A thin film transistor (TFT) is formed adjacent a cross point of the data bus line 2 and the gate bus line 1. The TFT, as shown in FIG. 1b which is a sectional view according to line I-I of FIG. 1a, includes a gate electrode 5, a gate insulator 12, a semiconductor layer 15, a channel layer 16, and source/drain electrode 6. The gate electrode 5 is connected to the gate bus line 1 and source/drain electrode 6 is connected to the data bus line 2. The gate insulator 12 is formed on the whole surface of the first substrate of the first substrate 10.
A common electrode 9 and a data electrode 8 are formed in the pixel region. The common electrode 9 is formed with the gate electrode 5 and connected to the common line 3. The date electrode 8 is formed with the source/drain electrode 6 and electrically connected to the source/drain electrode 6. Further, a passivation layer 20 and a first alignment layer 23a are deposited on the whole surface of the first substrate 10.
On a second substrate 11, a black matrix 28 is formed to prevent a light leakage which is generated around the TFT, the gate bus line 1, and the data bus line 2. A color filter layer 29, and a second alignment layer 23b is formed on the black matrix 28 in sequence. Also, a liquid crystal layer 30 is formed between the first and second substrates 10, 11.
When voltage is not applied to the LCD having the above structure, liquid crystal molecules in the liquid crystal layer 30 are aligned according to alignment directions of the first and second alignment layers 23a, 23b. However, when voltage is applied between the common electrode 9 and the data electrode 8, the liquid crystal molecules are vertically aligned to the extending directions of the common and data electrodes. In the foregoing, since liquid crystal molecules in the liquid crystal layer 30 are switched on the same plane at all times, grey inversion is not created in the up and down direction, and right and left direction of the viewing angle.
In the conventional LCD having the above structure, however, the aperture ratio is decreased by the data electrode 8 and the common electrode 9, which are opaque.